The symbol for the Heroic difficulty setting in the trilogy is represented with a shield that has the Marathon Logo encrusted into the center; over the shield are two UNSC Standard Issue Machetes crossed over each other. In Halo 3: ODST, it is represented by two M7S SMGs crossed over a shield with the Marathon symbol engraved into the center.

Heroic pits you against Covenant forces who are now slightly smarter, and their weapons inflict more damage per hit than their Normal-level counterparts. Even the weakest of infantry, the Grunts, can withstand quite a few bullets before they go down. While this setting would be considered the equivalent of "Hard" in other games, Bungie has stated that Heroic difficulty represents the "true Halo universe."

Higher ranked enemies start appearing more often (barring the Thunderstorm skull), giving the enemy AI more flexibility and strategy.

Enemy A.I. acts more intelligently. Jackals with Plasma Pistols will occasionally overcharge the pistol to instantly drain the player's shields, and enemies will hide more often when their health is low.

Increased enemy presence in several areas, typically being enemies that pose a major threat to the player. Such examples include the presence of Energy Sword using SangheiliCombat Forms who do not appear at all on lower difficulties.

Marines have reduced effectiveness and are weaker; their tactical intelligence has also been reduced. Due to this, their overall role in Heroic is to provide temporary diversions from enemies when you are being overwhelmed.

Ultra Elites and Ultra Grunts appear more often and Elites will commonly be armed with more powerful weapons or will dual-wield Plasma Rifles.

All of these combine into a decent gameplay option for more experienced players, who find Normal less exciting, and Legendary too challenging. This difficulty may be deemed the most realistic by some players; however, this is not to be stated as a fact, due to each individual having one's own comfort zone regarding difficulty.

In Halo 3, Heroic was intended for those who have played the prior games and multiplayer, resulting in increased skill. This increase means that those who found Normal comfortable in Halo 2, will find Heroic just right for Halo 3, according to Frankie. Martin O'Donnell stated that, 'Normal is for beginners who are embarrassed to pick Easy. Heroic is what we intended.'

Every difficulty shield, excluding Halo 4 and Reach, icon has the Marathon symbol on it.

Heroic difficulty is considered the "True Halo Universe" by most of the Bungie employees and creators, seeing that it mirrors reality of the Halo world in many ways.

Unlike other difficulties, the combat is more realistic. The enemy AI is better on higher difficulties, yet at the same time, friendly intelligence falls as well (which could be considered unrealistic in a sense since allies are already quite easy to kill).